Hmmm, that looks to me like internal reflections either within the lens itself, or from light bounced back to the lens from the surface of the sensor. Maybe this is a limitation of the 18-135mm lens. Have you seen it with other lenses?

Hmmm, so either the 50mm has some serious internal reflections or the last lens on the inside is not coated well enough so that it reflects the light that bounces back from the sensor (a well-known prob with DSLRs).
You can test this by dismounting the 50mm and look at the back-lens critically: does it reflect more than the front-lens? Compare it to the back-lens of your 18-135mm. If the 50mm "shines" on the backside, that is the most probable problem and is not going away.
If the backsides of both lenses are comparable (coating-wise), the reflection should be internal. In that case stopping down might reduce the prob. But then, what is the charm of a fast F1.8 lens?!

Again, you can see that contrast is more important than noise in modern DSLRs. See my rant here and vote!

Well if both have UV-filters of the same quality, that should not make the difference, but perhaps depending on the curvature of the front lens one lens is having more trouble with the additional reflective lens/air-surface than the other. If you want best contrast, just screw every filter off!

Bit late to the thread, sorry. I'd put money on it being the UV filter though. I'm sure something similar happened to me once while taking pictures of a refinery. and that was with film. Ah, those were the days...

On a lighter note, that shot of Auckland is most excellent. I honestly hope it is the filter, they're cheap. Relatively anyway.